The Towsley Fire broke out around 1:41 p.m. in the 23000 area of The Old Road, in the unincorporated Santa Susana Mountains, according to the Los Angeles County Fire Department.

By 4:45 p.m., the fire was 20 percent contained but forward progress was stopped, officials said.

The fire was sparked by a smoke grenade thrown at a nearby paintball park, according to Santa Clarita Sheriff's Sgt. Niels Gittisarn.

A 20-year-old man said he had gotten permission to use it from the park while a group of people played paintball, Gittisarn said. The paintball place, however, denied it.

The man was taken into custody and later arrested, sheriff's officials said.

The fire was not believed to have been set intentionally, according to Gittisarn.

Officials confirmed at least one outbuilding burned. Sky5 video over the scene showed a structure fully engulfed in flames.

Fifteen structures along The Old Road were at one point potentially threatened by the blaze, according to the Fire Department.

The U.S. Post office at 24201 The Old Rd. and the Newhall Church of the Nazarene at 23857 The Old Rd. were placed under a temporary evacuation order, Lt. Bauer of the Santa Clarita Sheriff's Station said.

The Old Road was shut down between Gavin Canyon and Calgrove Boulevard while crews battled the flames, according to a Fire Department news release. It was not known how long the closure would last.

The 5 Freeway is open in both direction in area, but traffic is jammed, officials warned.

Approximately 225 firefighting personnel were called in during the afternoon to battle the third-alarm fire on the ground and from the air, Fire Department Inspector Richard Lincon said.

Firefighters faced hot conditions as they tried to extinguish the growing blaze, with temperatures in the area reaching 89 degrees by 2:30 p.m., according to the National Weather Service. Forecasts called for a high of 94 degrees.

Winds with speeds of around 15 mph were blowing in a south-southwest direction, with expected gusts of up to 20mph, according to the weather service. Humidity was at 25 percent by mid-afternoon.